Observation:  I agree that the auto number field is the best primary key for a table but I'm not sure that it is always the best ID field.  If a person/thing is deleted that should not have been and they are reentered their id changes.  Their customer id would no longer be the same as the id given to the customer.   This is also a good reason to never delete.  I have tables with a unique primary key and a unique customer id.

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Reply from Alex Hedley:

You could add a boolean flag that says IsActive and just swap that, so if you ever need to bring the Customer back you can flip it again.